1. Field of the Invention
The present invention relates to a ferrule assembly and an optical module having the ferrule assembly, specifically serving as a light source of optical communication, optical fiber amplifiers, and the like.
2. Related Background Art
Laser modules are basically composed of a light-emitting device as a light source and an optical resonator comprising a pair of reflectors mutually reflecting the light emitted from the light-emitting device.
In the laser modules mentioned above, one of the reflectors constituting the resonator is one facet (back facet) of the light-emitting device itself or dielectric films having a high reflectivity formed on this back facet. A Bragg grating disposed near an end portion of the optical fiber opposing the light-emitting device is utilized as the other reflector. The Bragg grating is formed within the optical fiber by providing a periodic refractive index alteration, and reflects specific light determined by the Bragg diffraction condition. The optical fiber in which a Bragg grating is formed is described as grating faiber and is advantageous that the desirable oscillation wavelength can easily be selected by the specification of the Bragg grating.
In case of an optical resonator constituted by one facet of the light-emitting device and a grating fiber, a ferrule is attached to an end portion of the grating fiber, and a ferrule assembly constituted by the grating fiber and the ferrule is secured at a predetermined position. An optical coupling between the light-emitting device and the grating fiber can be realized by disposing an optical element (lens) between the facet of the light-emitting device and the end face of the grating fiber facing each other. The optical coupling can also be realized by rounding the facet of the grating fiber instead of providing the optical element.
In a typical configuration of the ferrule assembly, it is common that the optical axis of the light-emitting device and the grating fiber are aligned to each other. In order to prevent the light reflected by the front facet of the light-emitting device back into the waveguide of the light-emitting device again, however, the front facet of the light-emitting device is vertically inclined off the optical axis by a predetermined angle.
In case where the facet of the grating fiber is rounded, the ferrule assembly has such a configuration that the end portion of the grating fiber protrudes from the end surface of the ferrule. It is due to the fact that, while the facet of the light-emitting device and the end face of the grating fiber must be installed close to each other, it is structurally difficult for the ferrule to approach the light-emitting device. Hence, a part of the end of the grating fiber including the rounded facet thereof protrudes from the end surface of the ferrule.
As a result of detailed studies concerning the above-mentioned resonator, the inventor has found that the resonator characteristic is attributed to the structure of the ferrule assembly.
The first factor affecting to the resonator characteristic lies in tightly securing the whole portion of fiber grating to the inner wall of the ferrule with an adhesive. In this case, due to deformations, thermal expansions, and the like of the ferrule, a stress is directly applied to the fiber grating. The reflection characteristic of the fiber grating would change. The second factor is, when the grating fiber and the ferrule are not firmly fixed to each other, the installed position of the grating fiber may shift gradually due to the environment conditions. As a result, the coupling efficiency between the light-emitting device and the grating fiber deteriorates.